The effect of calcium as catalyst of the NO-carbon reaction has been investigated. Three carbons of different origin and surface properties were loaded by ion exchange with calcium acetate. The effect of catalyst loading was also investigated in one of the carbons. The samples were characterized by physical adsorption of CO2 (at 0 degrees C) and N-2 (at -196 degrees C) and by chemisorption of CO2 at 300 degrees C. The NO-carbon reaction was studied in a fixed-bed flow reactor at atmospheric pressure using two types of experiments : (i) temperature-programmed reaction in a NO/He mixture; and (ii) isothermal reaction at 300-600 degrees C. The reaction products were monitored in both cases, thus allowing detailed oxygen and nitrogen balances to be determined. Calcium was found to catalyze NO reduction by carbon through a mechanism that is consistent with the formation of intermediate CaO(O) surface species. Nevertheless, the calcium species present on the carbon surface before NO reduction (CaO) are much less effective than the potassium species (elemental potassium or potassium suboxide) in chemisorbing NO, as a result of which they transfer much less oxygen to the carbon active sites. The results show also that the porous structure and the surface chemistry of the carbon determine, in a complex way, the catalyst loading and dispersion, as well as the catalyst/substrate contact, and hence control the catalytic activity of calcium in NO reduction by carbon.